Rewrite:

- moved constants GRAVITY_* from boot.php to Friendica\Model\Item
- also rewrote some array initialization:

From:
````
<?php
$arr = [];
$arr['foo'] = "FOO";
````

To:
````
<?php
$arr['foo'] = "FOO";
````
- added a few type-hints

mod/tagger.php

@@ -123,7 +123,7 @@ EOT;
 	$arr['uid'] = $owner_uid;
 	$arr['contact-id'] = $contact['id'];
 	$arr['wall'] = $item['wall'];
-	$arr['gravity'] = GRAVITY_COMMENT;
+	$arr['gravity'] = Item::GRAVITY_COMMENT;
 	$arr['parent'] = $item['id'];
 	$arr['thr-parent'] = $item['uri'];
 	$arr['owner-name'] = $item['author-name'];